Underlayer for perpendicularly magnetized film, perpendicularly magnetized film structure, perpendicular MTJ element, and perpendicular magnetic recording medium using the same

ABSTRACT

Disclosed is a perpendicularly magnetized film structure using a highly heat resistant underlayer film on which a cubic or tetragonal perpendicularly magnetized film can grow, comprising a substrate of a cubic single crystal substrate having a (001) plane or a substrate having a cubic oriented film that grows to have the (001) plane; an underlayer formed on the substrate from a thin film of a metal having an hcp structure in which the [0001] direction of the thin metal film forms an angle in the range of 42° to 54° with respect to the &lt;001&gt; direction or the (001) orientation of the substrate; and a perpendicularly magnetized layer located on the metal underlayer and formed from a cubic material selected from a Co-based Heusler alloy and a cobalt-iron (CoFe) alloy having a bcc structure a constituent material, and grown to have the (001) plane.

TECHNICAL FIELD

The present invention relates to an underlayer for a perpendicularlymagnetized film using a non-magnetic material having a hexagonal closepacked structure, which serves as an underlayer intended for growing ofa ferromagnetic thin film, and to a perpendicularly magnetized filmstructure. Furthermore, the present invention relates to a perpendicularMTJ element and a perpendicular magnetic recording medium, which use therelevant perpendicularly magnetized film structure.

BACKGROUND ART

Along with the advances in high density recording and capacity increasewith respect to magnetic storages or memories, which are represented bymagnetic disk devices (hard disks) and magnetoresistive random accessmagnetic memories (MRAM) that use magnetic materials, utilization ofperpendicularly magnetized films that are magnetized in the directionperpendicular to the film plane is effective. For the increase in therecording density induced by miniaturization of the recording media forhard disks using this perpendicularly magnetized film or magnetic tunneljunction elements (MTJ elements) that constitute the recording bits ofMRAM, it is necessary to increase the magnetic anisotropy energy densityKu through quality improvement of the perpendicularly magnetized film.Also, in order to obtain a perpendicularly magnetized film with superiorquality, the presence of an underlayer that takes an important role inthe control of the crystal orientation or the crystal grain size,reduction of stacking faults, and securing of flatness, is extremelyimportant.

Non-Patent Literature 1 discloses that in perpendicular magnetizedrecording media of Co-based alloys such as a cobalt-platinum-chromium(Co—Pt—Cr) alloy or the like, a Ru underlayer having a hexagonal closepacked (hcp) structure is used, which has the same crystal structure asthese alloys do. Furthermore, in regard to a L1₀ type iron-platinum(FePt) alloy that is expected to be applied to recording media or MTJelements of the future because a very high Ku may be obtained,Non-Patent Literature 2 discloses magnesium oxide (MgO) having a sodiumchloride structure (NaCl structure) as an effective material for anunderlayer for the alloy, while Patent Literature 1 disclosesmagnesium-titanium oxide (MgTiO_(x)).

Furthermore, in a perpendicularly magnetized film for exclusive use in aMTJ element, perpendicular magnetization can be realized even for softmagnetic materials such as cobalt-iron-boron (CoFeB) or iron (Fe), whichdo not exhibit perpendicular magnetization in a bulk state, when theinterface effect of an ultrathin film structure is utilized. Therefore,it has been suggested that the perpendicularly magnetized film can beused as a recording layer (interface-induced perpendicularly magnetizedlayer). In this case, according to Non-Patent Literatures 3 and 4, amicrocrystalline material or a body-centered cubic (bcc) structure-basedmaterial, such as tantalum (Ta) or chromium (Cr), is utilized as anunderlayer.

However, the above conventional L1₀ type alloys and MgO underlayers havelattice mismatch at a proportion of close to 10%, and a flat film formhaving high crystallinity and high degree of order cannot be realized.Furthermore, the underlayers for conventional interface-inducedperpendicularly magnetized layers have poor heat resistance, and have aproblem that the heating treatment necessary for securing the tunnelmagnetoresistance (TMR) ratio of a MTJ element cannot be implemented.Also, some of ferromagnetic materials are subjected to the influence ofdistortion by the underlayer, and therefore, it is made impossible toextract sufficient characteristics. Therefore, it has been hithertodifficult to enhance the product quality of magnetic recording media orMTJ elements, which use these perpendicularly magnetized films.

CITATION LIST Patent Literature

-   Patent Literature 1: WO 2014/004398 A1

Non-Patent Literatures

-   Non-Patent Literature 1: Yoshihisa Nakamura (editor), “Advanced    Technologies of Perpendicular Magnetic Recording”, CMC Publishing    Co., Ltd. (2007/08)-   Non-Patent Literature 2: A. Perumal, Y. K. Takahashi, and K. Hono,    “L1₀ FePt—C Nanogranular Perpendicular Anisotropy Films with Narrow    Size Distribution,” Applied Physics Express, vol. 1, p. 101301    (2008).-   Non-Patent Literature 3: S. Ikeda et al., “A    perpendicular-anisotropy CoFeB—MgO magnetic tunnel junction”, Nature    Mater., vol. 9, pp. 721-724 (2010).-   Non-Patent Literature 4: Z. C. Wen, H. Sukegawa, S. Mitani, and K.    Inomata, “Perpendicular magnetization of Co₂FeAl full-Heusler alloy    films induced by MgO interface”, Applied Physics Letters, vol.    98, p. 242507 (2011).

SUMMARY OF INVENTION Technical Problem

The present invention solved the conventional problems in view of suchcircumstances, and it is an object of the present invention to provide aperpendicularly magnetized film structure that uses a highly heatresistant underlayer film on which a cubic or tetragonal perpendicularlymagnetized film can grow with high quality.

Furthermore, it is another object of the present invention to provide aperpendicularly magnetized film and a perpendicular MTJ element, both ofwhich are formed using the relevant perpendicularly magnetized filmstructure.

Solution to Problem

The inventors of the present invention found, in the course ofconducting a study on perpendicularly magnetized MTJ elements, a Ruunderlayer having a high crystal orientation index and having an hcpstructure, which is obtained on an MgO layer by controlling the growthconditions, and found that a cubic cobalt-iron-aluminum (Co₂FeAl) alloythin film grown on the Ru underlayer is formed with the (001)orientation and becomes a perpendicularly magnetized film. Furthermore,the present inventors also found that this perpendicularly magnetizedfilm has perpendicular magnetic anisotropy that is significantly highercompared to the case in which the perpendicularly magnetized film isproduced on a Cr layer, which is a general underlayer material, and thatin a case in which this perpendicularly magnetized film is used as aconstituent element for a MTJ element, an increase in the TMR ratio isalso obtained. Furthermore, the inventors also found that through thehighness of the melting point of Ru, and the difference in the crystalsystem between Ru and the alloy thin film, the perpendicularlymagnetized film has high heat resistance. The present inventors alsofound that in regard to rhenium (Re) that is a noble metal similarly toRu and has an hcp structure, rhenium grows with a high crystalorientation index that is equivalent to that of Ru on an MgO layer, andcan be utilized as an underlayer for a cubic ferromagnetic substance.This implies that this is widely effective not only for Ru, but also forany material having an hcp structure.

The present invention was completed based on these new findings.

That is, an underlayer for a perpendicularly magnetized film of thepresent invention includes a metal having an hcp structure, wherein a[0001] direction of the underlayer forms an angle in the range of 42° to54° with respect to a cubic single crystal substrate having a (001)plane or a cubic oriented film grown to have a (001) plane.

Here, the metal having an hcp structure may be of various kinds, andexamples of preferred metals include noble metals such as Ru and Re.

For example, in a case in which the metal is Ru, this is Ru having thehcp structure illustrated in FIG. 1 to FIG. 4(B). The Ru [0001]direction forms an angle in the range of 42° to 54° with respect to acubic single crystal substrate having the (001) plane or a cubicoriented film that grows in the (001) plane. In a case in which the Ru[0001] direction is less than 42°, the crystal orientation becomes lowerthan the (01-12) plane of Ru, and in a case in which the Ru [0001]direction is more than 54°, the crystal orientation becomes higher thanthe (03-34) plane of Ru. Accordingly, the tetragonal lattice of Ru doesnot appear, and therefore, Ru does not function as an underlayer forcubic and tetragonal perpendicularly magnetized films.

In the underlayer for a perpendicularly magnetized film of the presentinvention, at least one of the cubic single crystal substrate or thecubic oriented film is preferably formed from magnesium oxide ormagnesium-titanium oxide.

Furthermore, in regard to the underlayer for a perpendicularlymagnetized film of the present invention, the underlayer preferably hasa structure having any one of a (02-23) plane, a (03-35) plane and a(03-35) plane.

The perpendicularly magnetized film structure of the present inventionincludes, for example, as illustrated in FIG. 5, any one (5) of a cubicsingle crystal substrate having a (001) plane or a substrate having acubic oriented film grown to have a (001) plane; an underlayer (6)formed on the substrate 5, the underlayer including a metal thin filmhaving an hcp structure, in which the [0001] direction forms an angle inthe range of 42° to 54° with respect to the <001> direction or the (001)plane of the substrate 5; and a perpendicularly magnetized layer (7)located on the underlayer 6, the perpendicularly magnetized layer beingformed from a cubic material, as a constituent material, which isselected from the group consisting of a Co-based Heusler alloy, acobalt-iron (CoFe) alloy having a bcc structure, an L1₀-based alloy XY(X=Fe or Co, and Y=Pt or Pd), a DO₂₂ type or an L1₀ type manganesealloy, for example, a manganese-gallium (Mn—Ga) alloy or amanganese-germanium (Mn—Ge) alloy, and grows to have the (001) plane.

The perpendicularly magnetized film structure of the present inventionpreferably includes a non-magnetic layer (8) located on theperpendicularly magnetized layer.

The perpendicular MTJ element film of the present invention includes,for example, as illustrated in FIG. 6, any one (10) of a cubic singlecrystal substrate having a (001) plane or a substrate having a cubicoriented film grown in a (001) orientation; an underlayer (11) formed onthe substrate 10, the underlayer including a metal thin film having anhcp structure, in which a [0001] direction forms an angle in the rangeof 42° to 54° with respect to the <001> direction or the (001) plane ofthe substrate 10; a first perpendicularly magnetized layer (12) locatedon the underlayer 11, the first perpendicularly magnetized layer beingformed from a cubic material, as a constituent material, which isselected from the group consisting of a Co-based Heusler alloy, acobalt-iron (CoFe) alloy having a bcc structure, an L1₀-based alloy XY(X=Fe or Co, and Y=Pt or Pd), a DO₂₂ type or an L1₀ type manganesealloy, for example, a manganese-gallium (Mn—Ga) alloy and amanganese-germanium (Mn—Ge) alloy, and is grown in the (001)orientation; a tunnel barrier layer (13) located on the firstperpendicularly magnetized layer 12, the tunnel barrier layer containinga constituent material which is selected from the group consisting ofMgO, spinel (MgAl₂O₄), and aluminum oxide (Al₂O₃), and grows in the(001) orientation and a direction equivalent thereto; and a secondperpendicularly magnetized layer (14) located on the tunnel barrierlayer 13, the second perpendicularly magnetized layer being formed froma cubic material, as a constituent material, which is selected from thegroup consisting of a Co-based Heusler alloy, a cobalt-iron (CoFe) alloyhaving a bcc structure, an L1₀-based alloy XY (X=Fe or Co, and Y=Pt orPd), a DO₂₂ type or an L1₀ type manganese alloy, for example, amanganese-gallium (Mn—Ga) alloy and a manganese-germanium (Mn—Ge) alloy,and grows in the (001) orientation.

Preferably, the perpendicular MTJ element film may have an upperelectrode (15), which is preferably located on the secondperpendicularly magnetized layer 14 and contains Ta and at least one ofthe above-described metals as a constituent material.

The perpendicular magnetic recording medium of the present invention hasat least one of the underlayer for a perpendicularly magnetized film,the perpendicularly magnetized film structure, and the perpendicular MTJelement film.

The method for producing a perpendicularly magnetized film structure ofthe present invention includes a step of providing a cubic singlecrystal substrate 5 having a (001) plane; a step of forming a metal thinfilm having a hcp structure on the substrate 5; a step of forming ametal underlayer 6 by subjecting the metal thin film to a post-annealingtreatment in a vacuum at 200° C. to 600° C.; and a step of forming aperpendicularly magnetized layer 7 on the metal underlayer 6, using acubic material that is selected from the group consisting of a Co-basedHeusler alloy, a cobalt-iron (CoFe) alloy having a bcc structure, anL1₀-based alloy XY (X=Fe or Co, and Y=Pt or Pd), a DO₂₂ type or an L1₀type manganese alloy, for example, a manganese-gallium (Mn—Ga) alloy ora manganese-germanium (Mn—Ge) alloy, and grows to have the (001) plane.

The method for producing a perpendicular MTJ element film of the presentinvention includes a step of providing a cubic single crystal substrate10 having a (001) plane; a step of forming the metal thin film on thesubstrate 10; a step of forming a metal underlayer 11 by subjecting themetal thin film to a post-heating treatment in a vacuum at 200° C. to600° C.; a step of forming a first perpendicularly magnetized layer 12on the metal underlayer 11, using a cubic material which is selectedfrom the group consisting of a Co-based Heusler alloy, a cobalt-iron(CoFe) alloy having a bcc structure, an L1₀-based alloy XY (X=Fe or Co,and Y=Pt or Pd), a DO₂₂ type or an L1₀ type manganese alloy, forexample, a manganese-gallium (Mn—Ga) alloy or a manganese-germanium(Mn—Ge) alloy, and grows to have the (001) plane; a step of forming atunnel barrier layer 13 on the first perpendicularly magnetized layer12, the tunnel barrier layer containing a constituent material which isselected from the group consisting of MgO, spinel (MgAl₂O₄) and aluminumoxide (Al₂O₃), and grows in the (001) orientation and a directionequivalent thereto; and a step of forming a second perpendicularlymagnetized layer 14 on the tunnel barrier layer 13, the secondperpendicularly magnetized layer being formed from a cubic material, asa constituent material, which is selected from the group consisting of aCo-based Heusler alloy, a cobalt-iron (CoFe) alloy having a bccstructure, an L1₀-based alloy XY (X=Fe or Co, and Y=Pt or Pd), a DO₂₂type or an L1₀ type manganese alloy, for example, a manganese-gallium(Mn—Ga) alloy or a manganese-germanium (Mn—Ge) alloy, and grows to havethe (001) plane.

Preferably, the method for producing a perpendicular MTJ element filmmay have a step of forming an upper electrode 15, which is preferablylocated on the second perpendicularly magnetized layer 14 and containsTa and at least one of the above-described metals as a constituentmaterial.

Advantageous Effects of Invention

The fact that when a metal layer of ruthenium (Ru), rhenium (Re) or thelike, which have planes with high crystal indices, is realized, anatomic arrangement close to a tetragonal arrangement is obtained despitehaving a hexagonal close-packed (hcp) structure, is utilized, and aferromagnetic material belonging to the cubic system or the tetragonalsystem is enabled to grow in the (001) orientation. A perpendicularlymagnetized film having high heating resistance can be realized, andalso, a perpendicularly magnetized type perpendicular MTJ element usingthe perpendicularly magnetized film can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view diagram illustrating the basicstructure of an underlayer structure related to an embodiment of thepresent invention.

FIG. 2 is a diagram illustrating the relations between the Ru atomicarrangement, the crystal orientation, and the crystal planes of anunderlayer structure related to an embodiment of the present invention.

FIG. 3 is a diagram illustrating the relative relations between Ru grownin the (02-23) orientation and the substrate plane.

FIG. 4 shows a bird's-eye view diagram and a side view diagramillustrating the atomic arrangement of the Ru underlayer surface, inwhich FIG. 4(A) shows the (02-23) plane of Ru, and FIG. 4(B) shows the(03-34) plane of Ru.

FIG. 5 is a cross-sectional view diagram illustrating the basicstructure of a perpendicularly magnetized film structure related to anembodiment of the present invention.

FIG. 6 is a cross-sectional view diagram illustrating the basicstructure of a perpendicular magnetoresistance effect element structurerelated to an embodiment of the present invention.

FIG. 7(A) is a graph showing the magnetic characteristics of aperpendicularly magnetized film structure using a Ru underlayerstructure. FIG. 7(B) is a graph showing the magnetic characteristics ofa perpendicularly magnetized film structure using a Cr underlayerstructure, which is a conventional structure.

FIG. 8 is a graph showing a comparison between a Ru underlayer structureand a Cr underlayer structure as a conventional structure, in connectionwith the relations between the perpendicular magnetic anisotropy, Ku,and the heating treatment temperature, Tex.

FIG. 9(A) is a graph showing the relations between the magneticcharacteristics of a perpendicularly magnetized film structure using aRu underlayer structure and the CFA film thickness, t_(CFA). FIG. 9(B)is a graph in which the product of the perpendicular magneticanisotropy, Ku, and the CFA film thickness, t_(CFA), is plotted againstt_(CFA).

FIG. 10 is a graph in which the TMR ratio and the element resistance ofa perpendicular MTJ element using a Ru underlayer structure is plottedagainst an external magnetic field.

FIG. 11(A), FIG. 11(B) and FIG. 11(C) show X-ray diffraction patterns(CuKα radiation source) of a Ru underlayer structure/CFA 20 nm laminatestructure, and FIG. 11(A) is a graph showing the results of anout-of-plane scan, FIG. 11(B) is a graph showing the results of anin-plane scan in the [100] direction of an MgO substrate, while FIG.11(C) is a graph showing the results of an in-plane scan in the [110]direction of an MgO substrate.

FIGS. 12(A) and 12(B) show the results of X-ray pole scanning analysis(CuKα radiation source) of a Ru underlayer structure, and FIG. 12(A) isa graph showing the results for the (1-101) peaks of Ru, while FIG.12(B) shows the results for the (−2110) peaks of Ru.

FIG. 13(A) and FIG. 13(B) show the results of X-ray rocking curveanalysis (CuKα radiation source) of the (02-23) peaks of Ru of a Ruunderlayer structure, and FIG. 13(A) is a graph showing the results ofthe case without heating treatment, while FIG. 13(B) is a graph showingthe results of the case in which the heating temperature Tex=400° C.

FIG. 14 is a graph showing the X-ray film in-plane diffraction pattern(MoKα radiation source) of a Ru underlayer structure.

FIG. 15 is a graph showing the results of X-ray pole scanning analysis(CuKα radiation source) of the (202) peaks of CFA (20 nm) formed on a Ruunderlayer structure.

FIG. 16(A) and FIG. 16(B) show a bird's-eye view diagram and a side viewdiagram, respectively, that schematically reproduce the atomicarrangements of MgO and CFA on the (02-23) plane of Ru.

FIG. 17 is a diagram showing an atomic force microscopic image of thesurface of a Ru underlayer structure/CFA (1 nm) sample.

FIG. 18 is a diagram showing a high-angle scattering annular dark-fieldscanning transmission microscopic image of a perpendicular MTJ elementcross-section using a Ru underlayer structure.

FIG. 19(A) and FIG. 19(B) show a high resolution transmission electronbeam microscopic image of a perpendicular MTJ element cross-sectionusing a Ru underlayer structure, and FIG. 19(A) shows the direction ofMgO, while FIG. 19(B) shows the [110] direction of MgO.

FIG. 20(A) is a high resolution transmission type electron beammicroscopic image of the [110] direction of MgO in the vicinity of theinterface between an MgO substrate and a Ru underlayer structure. FIG.20(B) is a diagram schematically illustrating the Ru atomic arrangementand the (001) plane of MgO of FIG. 20(A). FIG. 20(C) is a graph obtainedby rotating FIG. 20(B) by 180° within the plane.

FIG. 21(A) and FIG. 21(B) show the X-ray diffraction pattern (CuKαradiation source) of a Ru underlayer structure/Fe 20 nm laminatestructure, and FIG. 21(A) is a graph showing the results of scanning inthe in-plane direction, while FIG. 21(B) is a graph showing the resultsof pole scanning of the (101) peaks.

FIG. 22 is a graph showing the film in-plane X-ray diffraction patterns(CuKα radiation source) of a Ru underlayer structure formed on an MgO(001) substrate, a MgAl₂O₄ (001) substrate, and a SrTiO₃ (001)substrate.

FIG. 23(A) to FIG. 23(F) show diagrams showing reflective high speedelectron beam diffraction images. FIG. 23(A) and FIG. 23(B) are imagesof a Re (30 nm) surface, FIG. 23(C) and FIG. 23(D) are images of a Fe(0.7 nm) surface, and FIG. 23(E) and FIG. 23(F) are images of an MgO (2nm) surface.

FIGS. 24(A) and 24(B) show the X-ray diffraction patterns (CuKαradiation source) of a Re underlayer structure/Fe (0.7 nm)/MgO (2 nm)laminate structure, and FIG. 24(A) is a graph showing the results ofscanning in the in-plane direction, while FIG. 24(B) is a graph showingthe results of pole scanning of the (0002) peaks of Re.

DESCRIPTION OF EMBODIMENTS

(A) Basic Structure

Hereinafter, an underlayer structure 1, a perpendicularly magnetizedfilm structure 4, and a perpendicular type magnetoresistance element(perpendicular MTJ element film 9) related to respective embodiments ofthe present invention will be described in detail with reference to FIG.1 to FIG. 6.

As illustrated in FIG. 1, the underlayer structure 1 according to anembodiment of the present invention comprises a substrate 2 and anunderlayer 3. The substrate 2 is a magnesium oxide (MgO) single crystalhaving a sodium chloride (NaCl) structure and the (001) plane direction.Furthermore, the substrate 2 may be an in-plane polycrystalline MgO filmoriented in the (001) orientation, or magnesium-titanium oxide(MgTiO_(x)) that has a NaCl structure and has an equivalent latticeconstant may also be used instead of MgO.

The underlayer 3 is formed from a metal such as ruthenium (Ru) orrhenium (Re), the [0001] direction (c-axis) of the crystal of the metalis inclined from a direction perpendicular to the film plane, so thatthe thin film surface has a high direction plane. For example, in a casein which the metal is ruthenium (Ru), as illustrated in FIG. 2, thecrystal planes of Ru include planes in the vicinity of from (01-12)orientation plane to the (03-34) orientation plane. The crystal planesform angles in the range of 42° to 54° with respect to the (0001) plane(c-plane), and for example, the (02-23) plane and the (03-35) plane areincluded in this range.

FIG. 3 illustrates, as a representative example, the relations betweenthe hexagonal close-packed lattice of Ru grown in the (02-23)orientation and the substrate plane in a schematic diagram. From thedifference in the crystal structure between MgO and Ru, a Ru layer isconfigured to include regions having different crystallographicdirections (variants), and in a case in which epitaxial MgO is used as asubstrate, the film has in-plane 4-fold symmetry as the whole film.

FIG. 4(A) schematically illustrates the crystal arrangement when the(02-23) plane is viewed from the Ru film top and from the sidedirection, and FIG. 4(B) similarly illustrates the crystal arrangementof the (03-34) plane. In FIG. 4(A) and FIG. 4(B), lattices of atomicarrangement aligned in a near square shape exist at the crystal surfacesof all Ru layers that function as underlayer 3. This square-shapedlattice is configured such that two layers of atomic planes present onthe (03-34) plane become a pair (for example, a pair of Ru1 and Ru2).The interatomic distance of a square-shaped lattice obtainable from thelattice constant of Ru (a=0.2704 nm and c=0.4278 nm) is 0.265 to 0.270nm, and the atomic plane distance in the diagonal direction is 0.189 nm.

Next, the perpendicularly magnetized film structure 4 according to anembodiment of the present invention will be described.

As illustrated in FIG. 5, the perpendicularly magnetized film structure4 has a substrate 5, an underlayer 6, a perpendicularly magnetized layer7, and a non-magnetic layer 8 laminated in this order. The substrate 5and the underlayer 6 are the same as the substrate 2 and the underlayer3, respectively, of the underlayer structure 1 in the case of FIG. 1.The perpendicularly magnetized layer 7 has a cubic material that growsto have the (001) plane, for example, a cobalt (Co)-based full Heusleralloy or a cobalt-iron (CoFe) alloy having a bcc structure(Co_(1-x)Fe_(x) (0≤x≤1)). A full Heusler alloy has a L2₁ type structureand has a chemical composition of Co₂YZ (wherein Y represents atransition metal, and Z mainly represents a typical element). The X andY atom sites are represented by, for example, X=Fe, Cr, Mn and alloysthereof, and Y=Al, Si, Ge, Ga, Sn, and alloys thereof. As a form of theCo-based full Heusler alloy, in addition to the L2₁ type, a B2structure, which is a structure in which the X and Y atom sites aredisorderly arranged, is also acceptable. Furthermore, CoFe alloys alsoinclude cobalt-iron-boron (CoFeB) alloys containing boron.

On the perpendicularly magnetized layer 7, in addition to the materialsdescribed above, a tetragonal material that can utilize thesquare-shaped lattice of Ru, for example, a L1₀-based alloy XY (X=Fe orCo, and Y=Pt or Pd), a DO₂₂ type or L1₀ type manganese alloy, forexample, a manganese-gallium (Mn—Ga) alloy or a manganese-germanium(Mn—Ge) alloy, are also capable of (001) growth, and therefore, thesematerials can be applied. It is because in these alloy materials, thelattice mismatch between such an alloy material and the Ru square-shapedlattice is as small as several percent (%) or less.

In a case in which a cubic material is used for the perpendicularlymagnetized layer 7, when the layer is formed into an ultrathin filmhaving a thickness of about 0.5 to 2 nm, and an oxide film of, forexample, MgO, is disposed as the non-magnetic layer 8, a perpendicularlymagnetized film is formed between the metal underlayer structure of Ru,Re or the like and the oxide film. In the case of a cubicperpendicularly magnetized film, this non-magnetic layer 8 is notnecessarily essential.

Next, the perpendicular MTJ element film 9 according to an embodiment ofthe present invention will be described. For example, as illustrated inFIG. 6, the perpendicular MTJ element film 9 includes a substrate 10, anunderlayer 11, a first perpendicularly magnetized layer 12, anon-magnetic layer 13, a second perpendicularly magnetized layer 14, andan upper electrode 15. The substrate 10, the underlayer 11, and thefirst perpendicularly magnetized layer 12 are the same as the substrate5, the underlayer 6, and the perpendicularly magnetized layer 7 of theperpendicularly magnetized film structure 4, respectively. The secondperpendicularly magnetized layer 14 is in direct contact with thenon-magnetic layer 13, and the same structure and material as those ofthe first perpendicularly magnetized layer 12 can be used. Furthermore,this layer may also include a perpendicularly magnetized film having anamorphous structure, for example, a terbium-cobalt-iron (Tb—Co—Fe) alloyfilm.

The non-magnetic layer 13 is an oxide layer, and is not only used forthe purpose of imparting perpendicular magnetic anisotropy, but alsofunctions as a tunnel barrier in the MTJ element. In the followingdescription, the non-magnetic layer 13 will be referred to as a tunnelbarrier layer. Regarding the tunnel barrier layer 13, MgO, spinel(MgAl₂O₄), or aluminum oxide (Al₂O₃) can be employed as the constituentmaterial, and the film thickness is from about 0.8 nm to 3 nm. In regardto MgAl₂O₄ and Al₂O₃, as long as the materials are cubic, they may havea structure with disorderly arranged cation sites. It is preferable thatthe tunnel barrier layer 13 grows in the (001) orientation and anorientation equivalent thereto. Thereby, the tunnel barrier layer 13together with the first perpendicularly magnetized layer 12 and thesecond perpendicularly magnetized layer 14 function as a MTJ element inthe (001) orientation, and therefore, a high TMR ratio is realized.

The upper electrode 15 is provided on the second perpendicularlymagnetized layer 14. The upper electrode 15 has a laminate structure ofmetals of tantalum (Ta)/Ru, Re or the like. The thicknesses of the eachlayer of Ta and the metal are, for example, 5 nm and 10 nm,respectively.

For example, due to its high melting point (2334° C.), Ru has a smallereffect of atomic diffusion caused by a heating treatment compared tochromium (Cr), which is a conventional material, and has improved heatresistance. Therefore, in a case in which Ru is used as the underlayer11, the constituent layers of a MTJ element or a magnetic recordingmedium layer can be subjected to a sufficient heating treatment forcharacteristics enhancement.

Furthermore, the Ru layer has an hcp structure, and has a crystalstructure that is different from that of the cubic and tetragonalperpendicularly magnetized layers. Therefore, the connection between therespective crystals is appropriately weakened, compared to the case of acombination of identical crystal structures. Thereby, the effect of thedistortion received from the underlayer can be weakened, and thecharacteristics of the perpendicularly magnetized layer can be enhancedby means of the production conditions. For example, for the MTJ elementof the present embodiment, the magnetic anisotropy Ku and the TMR ratiocharacteristics can be enhanced.

It is definitely needless to say that the metal having an hcp structureaccording to the present invention may be of various kinds includingrhenium (Re), in addition to ruthenium (Ru). For example, examplesthereof include Ru and Re, as well as noble metals such as osmium (Os)and rhodium (Rh), and alloys thereof; titanium (Ti), zirconium (Zr),hafnium (Hf), and zinc (Zn).

In a case in which the underlayer structure according to an embodimentof the present invention is used as a perpendicular magnetic recordingmedium, the underlayer structure and the perpendicularly magnetizedlayer require a thin film structure formed from microcrystal grainshaving aligned crystal orientations. On a thermally oxidized Sisubstrate having an amorphous structure or on a glass substrate, apolycrystalline film of MgO or MgTiO_(x) with (001) crystal orientationcan be produced by sputtering film formation, and the polycrystallinefilm can be used as an underlayer for the underlayer structure of thepresent embodiment. For example, a thermal oxide film-coated Sisubstrate/MgO/Ru/Co—Fe—Al alloy (Co₂FeAl):CFA structure can be utilized.

(B) Production Method

Hereinafter, the method for producing the underlayer structure 1, theperpendicularly magnetized film structure 4, and the perpendicular MTJelement film 9 according to the embodiments of the present inventionwill be described using FIG. 1, FIG. 5, and FIG. 6.

In the following, the production method will be explained by taking Ruas an example. First, regarding the method for producing a Ru layer asthe metal underlayers 3, 6 and 11, the substrates 2, 5 and 10 areproduced with MgO having the (001) plane, and film formation of a Ruthin film is performed by radiofrequency (RF) sputtering using anultrahigh vacuum magnetron sputtering apparatus (ultimate vacuum: about3×10⁻⁷ Pa). The thickness of the Ru film is, for example, 40 nm;however, if the film becomes a flat film, the film may be even thinner.Subsequently, a post-annealing treatment is performed at 200° C. to 600°C. in a vacuum, and thereby control of the crystal orientation plane isconducted. The angle formed at this time by the c-axis direction of Ruand the MgO substrate plane forms an angle in the range of 42° to 54°.

The CFA, which is a Co-based Heusler alloy, is formed on the Ruunderlayer. This CFA layer constitutes the perpendicularly magnetizedlayer 7 and the first perpendicularly magnetized layer 12. CFA is knownas a material having high spin polarization, and when CFA is used as aferromagnetic layer of a MTJ element, a very high TMR ratio can beobtained. CFA layer generally has a B2 structure, and there isirregularity between Fe sites and Al sites. As the degree of order of B2is higher, spin polarization is higher, and the TMR ratio thusobtainable becomes high. The CFA layer can be formed by sputtering filmformation from a Co—Fe—Al alloy target (molten target, representativecomposition 50:25:25 atomic %). The film thickness of the CFA layer isabout 0.5 to 1.5 nm, which is suitable for obtaining perpendicularmagnetization. For the CFA layer formation, a vacuum electron beamevaporation method or a co-sputtering method from plural targets can beutilized. At this time, the Ru square-shaped lattice is used as atemplate for crystal growth, and the (001) growth of cubic crystals isprompted. At the time of forming the CFA layer, when the substratetemperature is adjusted to 150° C., a B2 ordered structure is obtainedduring film forming, and also, flatness of the film can be secured. Inaddition to CFA, a material having a lattice constant that is close tothat of a cubic, for example, a Co-based Heusler alloy other than CFA,or a CoFe having a bcc structure can be used.

Next, an MgO layer as the tunnel barrier layer 13 is formed on the CFAlayer thus produced, so as to have a film thickness of, for example,about 1 to 2 nm. For the MgO film formation, direct RF sputtering filmformation from an MgO target, or a method of forming a film of magnesium(Mg) metal by sputtering and then subjecting the magnesium to anoxidation treatment, can be used. After the MgO layer is formed, thecrystal quality can be enhanced by performing a post-annealing treatmentat about 200° C. As the (001) orientation properties are enhanced, ahigher TMR ratio is obtained.

Subsequently, a CoFeB amorphous layer is formed as the secondperpendicularly magnetized layer 14 by sputtering film formation, andthe film thickness is adjusted to, for example, 1.3 nm. Then, forexample, Ta having a film thickness of 5 nm, and for example, a Ru layerhaving a film thickness of 10 nm are formed together thereon as theupper electrode 15 by sputtering film formation. The concentration ofboron (B) of the Co—Fe—B layer is decreased as boron undergoes atomicdiffusion into the Ta layer by a heating treatment, and therebycrystallization occurs from the MgO tunnel barrier layer. Thus, thestructure changes to a bcc structure having the (001) plane. Thereby, afirst perpendicularly magnetized layer 12/tunnel barrier layer 13/secondperpendicularly magnetized layer 14 structure grows in the (001)orientation, and therefore, a high TMR ratio is obtained. In order topromote this crystallization, a crystalline CoFe layer having athickness of 0.1 to 0.3 nm can be inserted between the MgO layer and theCoFeB layer.

(C) Characteristics

Next, the characteristics of the perpendicularly magnetized film of thepresent embodiment and a magnetoresistance effect element using theperpendicularly magnetized film will be explained in the followingExamples with reference to FIG. 7(A) to FIG. 10.

Example 1

(Perpendicular Magnetic Anisotropy)

An example of forming an MgO substrate/Ru/CFA/MgO structure as aperpendicularly magnetized film structure by sputtering film formationwill be described. In order to confirm the perpendicular magnetizationcharacteristics, the CFA film thickness was varied from 0.5 nm to 2.1 nmat an interval of 0.1 nm. The MgO film thickness was set to 1.8 nm. Forcharacteristics improvement, an annealing treatment in a vacuum wasperformed at a temperature in the range of Tex=250° C. to 450° C.

FIG. 7(A) shows a curve (magnetization curve) of magnetization (M) atroom temperature against an external magnetic field (H) for an MgOsubstrate/Ru/CFA/MgO structure in a case in which the CFA layer filmthickness, t_(CFA), was set to 1 nm, and the heating temperature, Tex,was set to 350° C. Both the out-of-plane direction (a directionperpendicular to the film plane) magnetic field and the in-planemagnetic field are shown. In the case of the out-of-plane directionmagnetic field, magnetization is easily inverted with respect to anexternal magnetic field, and magnetization is saturated in a smallmagnetic field; however, in the in-plane magnetic field, it is difficultto induce magnetization. Therefore, the structure has a largeperpendicular magnetic anisotropy. The perpendicular magnetic anisotropyenergy density (Ku) is a value corresponding to a region surrounded bythe curve for the out-of-plane magnetic field and the curve for thein-plane magnetic field, and the value means that in a case in whichthis area is large, the system has a large value of Ku. In FIG. 7(A),the value of Ku in the case of using the Ru underlayer of theembodiments of the present invention was 3.1×10⁶ emu/cm³.

FIG. 7(B) shows a magnetization curve in the case in which Cr, which isa conventional underlayer, was used as a Comparative Example. In theComparative Example, a laminate structure of Cr (40 nm)/CFA (1 nm)/MgO(2 nm) at an annealing temperature of Tex=350° C. is used. Aperpendicularly magnetized film is obtained also in this case; however,Ku was small compared to the case of using a Ru underlayer, and thevalue of Ku was 8×10⁵ emu/cm³. Therefore, according to the embodimentsof the present invention, by using a Ru underlayer instead of Cr ofComparative Example, the value of Ku increased to four times.

FIG. 8 shows the dependence of Tex on Ku in both cases of a Ruunderlayer and a Cr underlayer. At this time, the t_(CFA) was set to 1nm. A positive Ku in FIG. 8 represents a perpendicularly magnetizedfilm, and a negative Ku represents an in-plane magnetized film. In thecase of a Ru underlayer, perpendicular magnetization occurs over thewhole range of Tex of 250° C. to 400° C. On the other hand, in the caseof a Cr underlayer, perpendicular magnetization occurs only in a narrowrange of Tex of 300° C. to 350° C., and particularly, at Tex=400° C.,the value of Ku is markedly lowered. This is because the Cr layer andthe CFA layer diffused into each other, and the magnetic characteristicsof the CFA layer were significantly degraded. In the case of a Ruunderlayer, this underlayer always presents a larger value of Ku than aCr underlayer, and in the range of Tex described above, degradation ofperpendicular magnetic anisotropy is not observed. That is, it isimplied that in a case in which a Ru underlayer is used, theperpendicularly magnetized film can be adapted to an annealing treatmentat high Tex.

FIG. 9(A) shows a magnetization curve when the t_(CFA) is varied for anMgO substrate/Ru/CFA/MgO structure. From this diagram, it can be seenthat when the t_(CFA) is between 1.2 nm and 1.3 nm, the perpendicularlymagnetized film is converted to an in-plane magnetized film. FIG. 9(B)is a diagram in which the product of Ku and t_(CFA) at various Texvalues is plotted against t_(CFA). The term “As-depo.” means a samplethat was not subjected to an annealing treatment after the formation ofa film structure. In a case in which the product of Ku and t_(CFA) ispositive, this indicates perpendicular magnetization. Therefore, aperpendicularly magnetized film is obtained at Tex=300, 450° C. and at at_(CFA) in the range of 0.6 to 1.2 nm.

Meanwhile, perpendicular magnetization is hardly attained in theAs-depo. state; however, this is not due to the Ru underlayer, butbecause the quality of the crystal structure at the CFA/MgO interface isinsufficient.

The solid line in FIG. 9(B) is a straight line obtained by fitting usingthe following formula.Ku·t _(CFA)=(Kv−2πMs ²)t _(CFA) +Ks  (1)

Here, Ms represents saturation magnetization (in the case of a CGS unitsystem, unit: emu/cm³), Kv represents the crystal magnetic anisotropyenergy density (unit: erg/cm³), and Ks represents the interfaceanisotropy energy density at the MgO/CFA interface (unit: erg/cm²). Fromthe fitting calculation, Kv is negative, and the CFA layer itselfexhibits in-plane magnetic anisotropy in a case in which the MgO tunnelbarrier layer is absent. On the other hand, Ks is a segment of FIG.9(B), and is positive at any Tex. Therefore, the CFA layer becomes aperpendicularly magnetized film as a result of a quantum mechanicseffect at the interface between the CFA layer and the MgO tunnel barrierlayer. At Tex=350° C., the maximum value of Ks was 2.2 erg/cm². Thisvalue was twice or more compared to 1.0 erg/cm², which was a value inthe case of using a Cr underlayer in Comparative Example.

Example 2

(Magnetoresistance Effect)

As a MTJ element using a perpendicularly magnetized film, an MgOsubstrate/Ru (40 nm)/CFA (1.2 nm)/MgO (1.8 nm)/Fe (0.1 nm)/Co₂₀Fe₆₀B₂₀(1.3 nm)/Ta (5 nm)/Ru protection layer (10 nm) structure is described asan example. The annealing temperature Tex after the production of thefilm structure was set to 325° C.

FIG. 10 shows the results for the dependence of the magnetoresistancechange (TMR) ratio on an external magnetic field (H) in an out-of-planedirection plane at room temperature and a low temperature (10 K). Theblack and white arrows in the diagram represent the direction ofmagnetization of a CFA layer and a Fe/CoFeB layer, respectively. Sincesteep resistance changes with respect to the magnetic field areobserved, the CFA layer as the first perpendicularly magnetized layerand the Fe/CoFeB layer as the second perpendicularly magnetized layerboth become perfectly perpendicularly magnetized films, and it isimplied that a parallel magnetization state and an anti-parallelmagnetization state are realized in the range of the magnetic field usedfor analysis. The TMR ratio, which is defined as the tunnel resistancechange ratio at the time of the parallel magnetization state and at thetime of the anti-parallel magnetization state, was 132% at roomtemperature. This value is significantly large compared to 91% in thecase of using a Cr underlayer in Comparative Example. The TMR ratio atlow temperature was 237%. This increase in the TMR ratio caused by usinga Ru underlayer is mainly caused by factors such as that the influenceof Ru crystals is relatively small, and thereby quality enhancement ofthe CFA/MgO/Fe/CoFeB structure grown thereon was promoted, that thedegree of (001) orientation became high, and that the influence ofatomic diffusion between the laminate structure and the underlayer issmall.

(D) Crystal Structure

Next, the crystal structures for the underlayer structure and themagnetized film structure of the present embodiment will be explainedwith reference to FIG. 11(A) to FIG. 22.

FIG. 11(A), FIG. 11(B) and FIG. 11(C) show the results of X-raydiffraction using a copper (Cu) Kα radiation source for a sample inwhich a 20 nm CFA was produced on a 40-nm Ru underlayer on an MgOsubstrate. FIG. 11(A) shows the diffraction pattern in a case in whichX-radiation was scanned in the out-of-plane direction (2θ-ω scan); FIG.11(B) shows the diffraction pattern in a case in which X-radiationenters in parallel with the [100] direction of the MgO substrate, andscanning in the in-plane direction (2Θ_(X-φ) scan) was performed; andFIG. 11(C) shows the diffraction pattern of 2Θ_(X-φ) scan in a case inwhich X-radiation enters in the [110] direction of the MgO substrate. Itcan be seen from FIG. 11(A) that the diffraction peak originating fromRu is (02-23) only. In FIG. 11(B) and FIG. 11(C), diffraction peaksoriginating from the (1-101), (−2110) and (0-112) from the Ru layer areobserved in the 2Θ_(X-φ) scan. FIG. 12(A) and FIG. 12(B) show theresults of pole scan (ϕ scan) corresponding to the (1-101) plane and(−2110) plane, respectively. In both cases, peaks are obtained at aninterval of 90°, and thus, it is understood that the film is anepitaxial film having a 4-fold symmetry. This 4-fold symmetry indicatesthat the Ru in FIG. 3 comprises four variant regions that rotates 90°each within the plane of the film. Furthermore, it is also understoodfrom FIG. 12(A) that the (1-101) peaks have a structure split into two.This corresponds to the fact that (1-101) is slightly oblique from theMgO [100] direction, as will be explained below.

FIG. 13(A) and FIG. 13(B) show a rocking curve (co scan) of the (02-23)peak shown in the X-ray diffraction pattern in the out-of-planedirection, which was obtained in FIG. 12(A). Due to the existence ofvariants, in both cases of without an annealing treatment (as-depo.)(FIG. 13(A)) and Tex=400° C. (FIG. 13(B)), peaks exhibit a structuresplit into two. Through an annealing treatment, the (02-23) peaks of inthe diffraction pattern out of film plane become smaller, and the peaksof the rocking curve are clearly decomposed into two with an angle of2.94°. This implies that, as will be described below, the Ru layer wasrearranged into the (03-35) direction, which is a direction more optimalthan the (02-23) direction, through the heat treatment.

FIG. 14 shows the results of 2θ-ω scan performed using a molybdenum (Mo)Kα radiation source in order to obtain a diffraction pattern from highplane indices of a Ru underlayer. The sample used was 40-nm Ru grown onMgO, and the Tex was set to as-depo., 400° C., and 600° C. For all ofthe samples, it can be seen that the (02-23) peak appears, and at 600°C., (03-34) peak appears in addition to this (02-23) peak. Furthermore,since other Ru peaks are not observed, it was confirmed that a Ruunderlayer structure having crystal planes in the vicinity from (02-23)to the (03-34) was realized.

In the 2θ-ω scan (FIG. 11(A)) of the sample of Ru (40 nm)/CFA (20 nm),it can be seen that only (002) and (004) are observed as the peaks ofthe CFA layer, and the CFA layer grows in the (001) orientation. Fromthe intensity ratio of the (002) peak and the (004) peak, the degree oforder of B2 was calculated, and the calculated value was almost equal tothe theoretical value. Therefore, it was found that CFA had a nearlyperfect B2 structure. Furthermore, FIG. 15 shows the results of ϕ scanin the (202) plane of CFA. The 4-fold symmetry originating from the B2structure of CFA is observed, and CFA is obtained as an epitaxial film.Therefore, a Ru layer that grows with a highly crystalline plane isacknowledged to be effective as an underlayer for a high-quality CFAfilm with a B2 order.

A result obtained by schematically reproducing the atomic arrangementsof an MgO substrate and a CFA film on the (02-23) plane of Ru based onthe results described above, is presented in FIGS. 16(A) and 16(B). Aplane intersecting the crystal plane shown in the X-ray diffractionpattern is also shown. It can be seen that the square-shaped lattice ofRu overlaps with the lattices of MgO and CFA. Furthermore, the reasonwhy the peaks of pole scan of (1-101) in FIG. 12(A) have a structuresplit into two, is that the intersection plane of (1-101) of Ru has agradient of 40.5° in the [110] direction of MgO, that is, 4.5° in the[100] direction of MgO.

FIG. 17 shows the results of observing the surface of a sample having anMgO substrate/Ru (40 nm, annealed at 400° C.)/CFA (1 nm) structure usingan atomic force microscope (AFM). The average roughness Ra is 0.24 nm,which means flatness; however, it is seen that the surface has anundulation having a size of about 30 nm. In order to clarify therelation between this undulation and Ru, and the structure, the resultsobtained by observing a cross-section of a perpendicular MTJ elementfilm having an MgO substrate/Ru (40 nm)/CFA (1.2 nm)/MgO (1.8 nm)/Fe(0.1 nm)/Co₂₀Fe₆₀B₂₀ (1.3 nm)/Ta (5 nm)/Ru protection layer (10 nm)structure (Tex=325° C.), using a high-angle scattering annulardark-field scanning transmission microscope (HAADF-STEM), are shown inFIG. 18. A dark region represents a CFA/MgO/CoFeB structure. It is alsounderstood that the surface of the Ru layer has an undulation having aperiod of about 30 nm similarly to the AFM results, and this undulationperiod is approximately the same as the undulation period of the MgOsubstrate. Since the local diffraction image pattern obtained bynanobeam electrons generally varies with the undulation, it can be seenthat the period of the undulation is correlated to the domain size ofvariants. From the above results, the undulation structure is related tothe surface unevenness of the substrate and the domain size of thevariant; however, the undulation structure is not a disorder in thestructure in an atomic scale that affects the perpendicular magneticanisotropy or the TMR ratio.

Furthermore, in order to definitely clarify the Ru structure, anobservation of high-resolution transmission electron beam microscope(HRTEM) images was performed at cross-sections in the [100] directionand the [110] direction with respect to an MgO substrate. FIG. 19(A)shows an HRTEM image of the vicinity of the CFA layer in the [100]direction of MgO, and FIG. 19(B) shows an HRTEM image of the vicinity ofthe CFA layer in the [110] direction of MgO. From FIG. 19(A), crossstripes in the in-plane direction of Ru are observed, and the spacing isabout 0.19 nm. This value is almost consistent with 0.189 nm, which isthe atomic plane spacing predicted along the diagonal of thesquare-shaped array of Ru that is schematically illustrated in FIG.4(A). Furthermore, the crystal lattice of CFA is extended for about 3%in the in-plane direction due to the presence of an MgO tunnel barrierlayer, and it can be seen that there is a non-negligible tetragonaldistortion. This distortion under tensile force in the in-planedirection has a function of weakening the perpendicular magneticanisotropy.

However, it is shown that when the Ru underlayer of the presentembodiment is used, the effect of increasing the perpendicular magneticanisotropy as a result of quality enhancement of a CFA/MgO interfacialstructure dominates this cubic distortion. At the same time, it isimplied that the Ru underlayer functions not only as an underlayer forcubic crystals but also as an underlayer for tetragonal crystals.

FIG. 19(B) shows the direction of observation corresponding to FIG. 2,and the same atomic arrangement is predicted. As illustrated with dotsin FIG. 19(B), the atomic arrangement is consistent with the atomicarrangement of the vicinity of the (02-23) plane of Ru. From these, itcan be seen that the structure obtained by X-ray diffraction ismaintained even in very local regions. It was also confirmed that aperpendicularly magnetized CFA layer has a B2 structure. Furthermore, itwas also confirmed that the MgO layer had a NaCl structure, and grew inthe (001) orientation. Thus, it is valid to obtain a large Ku and a highTMR ratio.

In order to confirm the direction of growth of Ru with respect to an MgOsubstrate, an HRTEM image (MgO [110] direction) of the vicinity of asubstrate having an underlayer structure of MgO substrate/Ru (40 nm,Tex=400° C.) is shown in FIG. 20(A). It can be seen that the angleformed by the MgO (001) plane and the Ru (0001) plane is about 47°. Thisrelation is schematically illustrated in FIG. 20(B), and this anglecorresponds to the fact that Ru grows to have the (03-35) orientation.The (03-35) plane is a highly directional plane existing between the(01-12) plane and the (02-23) plane. Since (03-35) is a forbidden linefor X-radiation and electron beams, (03-35) cannot be identifieddirectly by X-ray diffraction and electron beam diffraction. However,(03-35) can be indirectly identified from a rocking curve of (02-23) ofFIG. 13(B). First, a peak that has split into two represents diffractionfrom another variant that underwent 180° in-plane rotation, as comparedto FIG. 20(B) and FIG. 20(C), and indicates that the peaks are growingin a direction inclining by an increment of 4=2.94° each from the(02-23) direction. Furthermore, the intensity at the center of the splitpeak becomes weak, and this indicates that the crystal direction of theRu underlayer is oblique from (02-23). The calculated value of the angleformed by the (02-23) plane and the (03-35) plane was 2.99°, and this isalmost consistent with Δ=2.94° obtained in FIG. 13(B). Therefore, it isconcluded that a Ru underlayer that was subjected to a heating treatmentat Tex=400° C. has a highly directional plane of the (03-35) plane inboth HRTEM and X-ray diffraction.

From the structural analysis described above, it was found thatregarding Ru, the (02-23), (03-35) and (03-34) planes are obtained bythe annealing treatment temperature, and rearrangement occurs in anoptimal plane. All of the crystal planes function effectively as anunderlayer for a cubic crystal material due to the presence ofsquare-shaped lattices at the surface of the Ru underlayer.

Example 3

Next, the X-ray diffraction pattern obtained by forming iron (Fe) havinga bcc structure on Ru as a ferromagnetic layer is shown in FIG. 21(A)and FIG. 21(B). The thickness of the Fe layer was set to 20 nm. From the2θ-ω scan of FIG. 21(A), only the (002) peak is obtained in the Felayer, and the Fe layer is formed by growing in the (001) planedirection, similarly to the CFA layer. Furthermore, from ϕ scanning ofFe (101) of FIG. 21(B), 4-fold symmetric peaks are observed, andepitaxial growth can be identified. Therefore, Ru effectively functionsas an underlayer also for cubic materials other than CFA.

Example 4

Next, in order to confirm the influence of an MgO substrate, 40-nm Ruwas formed using single crystal substrates of cubic SrTiO₃ (latticeconstant: 0.385 nm) and MgAl₂O₄ (lattice constant 0.808 nm), which havelattice constants different from that of MgO (lattice constant 0.421nm). The results of 2θ-ω scan are shown in FIG. 22. For all of thesubstrates, the (02-23) peak of Ru is observed; however, Ru (0002) andRu (0004) peaks that are not identified in MgO are observed at the sametime. Therefore, both the (0001) growth and the high index plane growthare available together. Therefore, it is understood that for the growthof a Ru underlayer, it is more preferable to use MgO.

(E) Summary

A comparison is made in Table 1 in connection with the difference in thecrystal structure between the Ru underlayer structure of the presentembodiment and a Cr underlayer structure, which is a conventionalstructure, and a comparison is also made for the perpendicular magneticcharacteristics and the TMR ratios of CFA perpendicularly magnetizedfilms constructed using those underlayer structures.

TABLE 1 Cr underlayer Item Ru underlayer structure structure Crystalstructure Hexagonal close-packed Body-centered structure cubic structure— — — Crystal plane (0223), (0335), (0334) (001) Presence or absence ofPresent (4 kinds) Absent variant Heat-resistant temperature >450 350 (°C.) Perpendicular magnetic 3.1 × 10⁶ 8.0 × 10⁵ anisotropy K_(u)(erg/cm³) (for 1 nm CFA) Interfacial magnetic 2.2 1.0 anisotropy K_(s)(erg/cm²) TMR ratio (%) at room 132 91 temperature

In a Ru underlayer structure, high perpendicular magnetic anisotropy anda high TMR ratio are realized irrespective of the fact that the crystalstructure is complicated. When the Ru underlayer structure has high heatresistance in addition to the high characteristics required from theseperpendicular MTJ elements, the adverse effect of an annealing treatmentduring the production process for memory elements including MRAM can besuppressed. Furthermore, in a case in which the Ru underlayer structureis used as an underlayer structure in a magnetic recording medium, anannealing treatment that is necessary to increase the degree of alloydisorder and to obtain strong perpendicular magnetization can be carriedout.

Example 5

As an example of using an element other than Ru, having an hcpstructure, as the underlayer, the growth of a Re underlayer will beexplained. Re is a noble metal having an hcp structure similarly to Ru.The lattice constants are a=0.2761 nm and c=0.4458 nm, and in a case inwhich Re grows to have the (02-23) plane, the interatomic distance in asquare-shaped lattice that is equivalent to FIG. 4(A) is 0.274 to 0.276nm, and the atomic plane spacing in a diagonal direction is 0.1195 nm.Since these are larger by about 2% than that of Ru, lattice matchingproperties with MgO are further improved. Furthermore, since the meltingpoint is very high such as 3186° C., growth equivalent to a Ruunderlayer and a heat resistant effect are expected. Thus, a Re (30nm)/Fe (0.7 nm)/MgO (2 nm) structure was produced on an MgO (001)substrate using a RF magnetron sputtering apparatus. The Re layer wasformed at room temperature under the conditions of a RF power of 50 Wand an Ar process gas pressure of 0.2 Pa, and then a post-annealingtreatment in a vacuum at 300° C. was performed. The Fe and MgO layerswere produced under the same conditions as those for Re, using RFsputter.

FIGS. 23(A) and 23(B) are images obtained by performing an observationof the surface immediately after the formation of a Re (30 nm) filmusing a reflection high-energy electron diffraction (RHEED). Streak-likeimages were obtained, and epitaxial growth was confirmed. Furthermore,the Re film has a 4-fold symmetry with respect to the substraterotation, and it was also found that an image that is almost equivalentto the case of a Ru underlayer is obtained. Next, FIGS. 23(C) and 23(D)show RHEED images after the formation of a Fe (0.7 nm) film, and FIGS.23(E) and 23(F) show RHEED images after the formation of an MgO (2 nm)film. Since the Fe layer undergoes epitaxial growth along the Reunderlayer structure, and (001) growth with a bcc structure, it wasconfirmed that the Re underlayer also functions as an underlayer forcubic ferromagnetic materials. Also, it was confirmed that the MgO layerunderwent epitaxial growth.

FIG. 24(A) shows the X-ray diffraction profile (2θ-ω scan) of amultilayer film having this MgO substrate/Re (30 nm)/Fe (0.7 nm)/MgO (2nm) structure. Clear (02-23) peaks are observed at near 2θ=110°.Furthermore, (0002) and (0004) peaks are observed, but their intensitiesare relatively weak, and (02-23) planar growth is the main component. Inorder to identify (02-23) orientation growth and epitaxial growth, thesubstrate may be inclined to the incident direction of X-radiation, andϕ scanning may be performed with respect to the (0002) peaks. FIG. 24(B)shows the results of ϕ scanning with respect to the (0002) peaks in thecase of inclining the substrate by 49.5°. From the results that 4-foldsymmetric peaks are clearly obtained, and that the angle of the inclinedsubstrate (49.5°) is close to the angle formed by the (0001) plane andthe direction of the (02-23) plane (50.6°, calculated value), it can beconcluded that epitaxial growth in the direction of the (02-23) plane ofRe has been realized.

From the above results, it was confirmed that Re also has the (02-23)plane of a high crystal orientation index and is capable of epitaxialgrowth similarly to Ru, and functions as an underlayer for a cubicferromagnetic layer.

INDUSTRIAL APPLICABILITY

The perpendicularly magnetized film according to the present inventioncan be utilized as a perpendicular magnetic recording medium, and isparticularly suitable to be used for perpendicular magnetic recordingdisks that are mounted in magnetic disk apparatuses such as HDD.Furthermore, the perpendicularly magnetized film may be particularlysuitably used as a discrete track medium (DTM) or a bit-patterned medium(BPM), which are promising as media for realizing ultrahigh recordingdensities that surpass the data recording densities of the currentperpendicular magnetic recording media, or as a medium exclusively forthermally assisted magnetic recording that can achieve ultrahighrecording densities that surpass the data recording densities obtainableby a perpendicular magnetic recording system.

REFERENCE SIGNS LIST

-   -   1 UNDERLAYER STRUCTURE    -   2, 5, 10 SUBSTRATE    -   3, 6, 11 UNDERLAYER    -   4 PERPENDICULARLY MAGNETIZED STRUCTURE    -   7 PERPENDICULARLY MAGNETIZED LAYER    -   8, 13 NON-MAGNETIC LAYER    -   9 PERPENDICULAR MTJ ELEMENT    -   12 FIRST PERPENDICULARLY MAGNETIZED LAYER    -   14 SECOND PERPENDICULARLY MAGNETIZED LAYER    -   15 UPPER ELECTRODE

The invention claimed is:
 1. An underlayer for a perpendicularlymagnetized film comprising a metal having an hcp structure, wherein a[0001] direction of the underlayer forms an angle in the range of 42° to54° with respect to a cubic single crystal substrate having a (001)plane or a cubic oriented film grown to have a (001) plane.
 2. Theunderlayer for a perpendicularly magnetized film according to claim 1,wherein at least one of the cubic single crystal substrate or the cubicoriented film is formed from magnesium oxide or magnesium-titaniumoxide.
 3. The underlayer for a perpendicularly magnetized film accordingto claim 1, wherein the metal has a structure having any one of a(02-23) plane, a (03-35) plane and a (03-34) plane.
 4. The underlayerfor a perpendicularly magnetized film according to claim 1, wherein themetal is at least one kind of noble metal.
 5. The underlayer for aperpendicularly magnetized film according to claim 4, wherein the noblemetal is ruthenium (Ru) or rhenium (Re).
 6. A perpendicular MTJ elementfilm comprising: any one of a cubic single crystal substrate having a(001) plane, or a substrate having a cubic oriented film grown to have a(001) plane; an underlayer formed on the substrate and formed from ametal thin film having an hcp structure, in which a [0001] direction ofthe metal forms an angle in the range of 42° to 54° with respect to the<001> direction or the (001) plane of the substrate; a firstperpendicularly magnetized layer located on the metal underlayer andformed from a cubic crystal material as a constituent material selectedfrom the group consisting of a Co-based Heusler alloy, a cobalt-iron(CoFe) alloy having a bcc structure, an L1₀-based alloy XY, wherein X isFe or Co and Y is Pt or Pd, a DO₂₂ type manganese alloy, and an L1₀ typemanganese alloy, and grown in the (001) orientation; a tunnel barrierlayer located on the first perpendicularly magnetized layer andcontaining a constituent material selected from the group consisting ofMgO, spinel (MgAl₂O₄), and aluminum oxide (Al₂O₃), and grown in the(001) orientation and a direction equivalent thereto; and a secondperpendicularly magnetized layer located on the tunnel barrier layer andformed from a cubic material selected from the group consisting of aCo-based Heusler alloy, a cobalt-iron (CoFe) alloy having a bccstructure, an L1₀-based alloy XY, wherein X is Fe or Co and Y is Pt orPd, a DO₂₂ type manganese alloy, and an L1₀ type manganese alloy, andgrown in the (001) orientation.
 7. The perpendicular MTJ element filmaccording to claim 6, wherein the metal thin film is at least one kindof noble metal thin film.
 8. The perpendicular MTJ element filmaccording to claim 7, wherein the noble metal thin film is a ruthenium(Ru) thin film or a rhenium (Re) thin film.
 9. The perpendicular MTJelement film according to claim 6, wherein the first perpendicularlymagnetized layer located on the metal underlayer is formed from a cubiccrystal material as a constituent material selected from the groupconsisting of a DO₂₂ type manganese-gallium (Mn—Ga) alloy, a DO₂₂ typemanganese-germanium (Mn—Ge) alloy, an L1₀ type manganese-gallium (Mn—Ga)alloy, and an L1₀ type manganese-germanium (Mn—Ge) alloy, and the secondperpendicularly magnetized layer located on the tunnel barrier layer isformed from a cubic material selected from the group consisting of aDO₂₂ type manganese-gallium (Mn—Ga) alloy, a DO₂₂ typemanganese-germanium (Mn—Ge) alloy, an L1₀ type manganese-gallium (Mn—Ga)alloy, and an L1₀ type manganese-germanium (Mn—Ge) alloy.
 10. Aperpendicular magnetic recording medium comprising at least one of: (i)the underlayer for a perpendicularly magnetized film according to claim1, the perpendicularly magnetized film structure comprising any one of acubic single crystal substrate having a (001) plane, or a substratehaving a cubic oriented film grown to have a (001) plane; an underlayerformed on the substrate and formed from a metal thin film having an hcpstructure, in which a [0001] direction of the metal forms an angle inthe range of 42° to 54° with respect to the <001> direction or the (001)plane of the substrate; and a perpendicularly magnetized layer locatedon the metal underlayer and formed from a cubic material as aconstituent material selected from the group consisting of a Co-basedHeusler alloy, a cobalt-iron (CoFe) alloy having a bcc structure, anL1₀-based alloy XY, wherein X is Fe or Co and Y is Pt or Pd, a DO₂₂ typemanganese alloy, and an L1₀ type manganese alloy, and grown in the (001)orientation, and (ii) the perpendicular MTJ element film according toclaim 6 comprising any one of a cubic single crystal substrate having a(001) plane, or a substrate having a cubic oriented film grown to have a(001) plane; an underlayer formed on the substrate and formed from ametal thin film having an hcp structure, in which a [0001] direction ofRu forms an angle in the range of 42° to 54° with respect to the <001>direction or the (001) plane of the substrate; a first perpendicularlymagnetized layer located on the metal underlayer and formed from a cubiccrystal material as a constituent material selected from the groupconsisting of a Co-based Heusler alloy, a cobalt-iron (CoFe) alloyhaving a bcc structure, an L1₀-based alloy XY, wherein X is Fe or Co andY is Pt or Pd, a DO₂₂ type manganese alloy, and an L1₀ type manganesealloy, and grown in the (001) orientation; a tunnel barrier layerlocated on the first perpendicularly magnetized layer and containing aconstituent material selected from the group consisting of MgO, spinel(MgAl₂O₄), and aluminum oxide (Al₂O₃), and grown in the (001)orientation and a direction equivalent thereto; and a secondperpendicularly magnetized layer located on the tunnel barrier layer andformed from a cubic material selected from the group consisting of aCo-based Heusler alloy, a cobalt-iron (CoFe) alloy having a bccstructure, an L1₀-based alloy XY, wherein X is Fe or Co and Y is Pt orPd, a DO₂₂ type manganese alloy, and an L1₀ type manganese alloy, andgrown in the (001) orientation.
 11. The perpendicular magnetic recordingmedium according to claim 10, wherein the first perpendicularlymagnetized layer located on the metal underlayer is formed from a cubiccrystal material as a constituent material selected from the groupconsisting of a DO₂₂ type manganese-gallium (Mn—Ga) alloy, a DO₂₂ typemanganese-germanium (Mn—Ge) alloy, an L1₀ type manganese-gallium (Mn—Ga)alloy, and an L1₀ type manganese-germanium (Mn—Ge) alloy, and the secondperpendicularly magnetized layer located on the tunnel barrier layer isformed from a cubic material selected from the group consisting of aDO₂₂ type manganese-gallium (Mn—Ga) alloy, a DO₂₂ typemanganese-germanium (Mn—Ge) alloy, an L1₀ type manganese-gallium (Mn—Ga)alloy, and an L1₀ type manganese-germanium (Mn—Ge) alloy.
 12. A methodfor producing the perpendicular MTJ element film according to claim 6,the method comprising: a step of providing a cubic single crystalsubstrate having a (001) plane; a step of performing film formation of ametal thin film on the substrate; a step of subjecting the metal thinfilm to a post-annealing treatment in a vacuum at 200° C. to 600° C.,and thereby forming a metal underlayer; a step of forming a firstperpendicularly magnetized layer formed on the metal underlayer, andformed from a cubic material as a constituent material selected from thegroup consisting of a Co-based Heusler alloy, a cobalt-iron (CoFe) alloyhaving a bcc structure, an L1₀-based alloy XY, wherein X is Fe or Co andY is Pt or Pd, a DO₂₂ type manganese alloy, and an L1₀ type manganesealloy, and grown to have the (001) plane direction; a step of forming atunnel barrier layer on the first perpendicularly magnetized layer andcontaining a constituent material selected from the group consisting ofMgO, spinel (MgAl₂O₄) and aluminum oxide (Al₂O₃) and grown in the (001)orientation and an orientation equivalent thereto; and a step of forminga second perpendicularly magnetized layer formed on the tunnel barrierlayer, and formed from a cubic material selected from the groupconsisting of a Co-based Heusler alloy, a cobalt-iron (CoFe) alloyhaving a bcc structure, an L1₀-based alloy XY, wherein X is Fe or Co andY is Pt or Pd, a DO₂₂ type manganese alloy, and an L1₀ type manganesealloy, and grown to have the (001) plane.
 13. The method for producing aperpendicular MTJ element according to claim 12, wherein the metal thinfilm is at least one kind of noble metal thin film.
 14. The method forproducing a perpendicular MTJ element according to claim 13, wherein thenoble metal thin film is a ruthenium (Ru) thin film or a rhenium (Re)thin film.
 15. The method according to claim 12, wherein the step offorming the first perpendicularly magnetized layer located on the metalunderlayer is formed from a cubic crystal material as a constituentmaterial selected from the group consisting of a DO₂₂ typemanganese-gallium (Mn—Ga) alloy, a DO₂₂ type manganese-germanium (Mn—Ge)alloy, an L1₀ type manganese-gallium (Mn—Ga) alloy, and an L1₀ typemanganese-germanium (Mn—Ge) alloy, and the step of forming the secondperpendicularly magnetized layer located on the tunnel barrier layer isformed from a cubic material selected from the group consisting of aDO₂₂ type manganese-gallium (Mn—Ga) alloy, a DO₂₂ typemanganese-germanium (Mn—Ge) alloy, an L1₀ type manganese-gallium (Mn—Ga)alloy, and an L1₀ type manganese-germanium (Mn—Ge) alloy.